(1) Field of the Invention
The present invention relates generally to positive photoresists containing cresol-aldehyde novolak resins having a high p-cresol content and a photoactive component comprising mono-, di-, and triesters of 2,3,4-trihydroxybenzophenone and 1,2-naphthoquinone-2-diazo-4-sulfonyl chloride.
(2) Description of the Prior Art
It is well known in the art to produce positive photoresist formulations such as those described in U.S. Pat. Nos. 3,666,473; 4,115,128; and 4,173,470. These include alkali-soluble phenol- (or cresol-) formaldehyde novolak resins together with light-sensitive materials (sometimes called "photoactive components"), usually substituted naphthoquinonediazide compounds. The resins and sensitizers are dissolved in an organic solvent or mixture of solvents and are applied as a thin film or coating to a substrate suitable for the particular application desired.
The novolak resin component of these positive photoresist formulations is soluble in aqueous, alkaline solutions, but the naphthoquinone sensitizer acts as a dissolution rate inhibitor with respect to the resist. Upon exposure of selected areas of the coated substrate to actinic radiation, however, the sensitizer undergoes a radiation induced structural transformation, and the exposed areas of the photoresist coating are rendered more soluble than the unexposed areas. This difference in solubility rates causes the exposed areas of the positive photoresist coating to be dissolved when the substrate is immersed in alkaline developing solution while the unexposed areas are largely unaffected, thus producing a positive relief pattern on the substrate.
In most instances, the exposed and developed substrate will be subjected to treatment by a substrate etchant solution or by dry etching. The positive photoresist coating protects the coated areas of the substrate from the etchant and thus the etchant is only able to etch the uncoated areas of the substrate, which in the case of a positive photoresist, correspond to the areas that were exposed to actinic radiation. Thus, an etched pattern can be created on the substrate which corresponds to the pattern of the mask, stencil, template, or the like that was used to create selective exposure patterns on the coated substrate prior to development.
The relief pattern of photoresist on substrate produced by the method described above is useful for various applications including, for example, as an exposure mask or pattern such as is employed in the manufacture of miniaturized integrated electronic components.
The properties of a photoresist composition which are important in commercial practice include the photospeed of the resist, development contrast, resist resolution, and resist adhesion. Also, since the photoresist may have to withstand an ion implantation process which may be conducted at high temperature, the thermal stability of the photoresist may be important.
Increased photospeed is important for a photoresist, particularly in applications where a number of exposures are needed, for example, in generating multiple patterns by a repeated process, or where light of reduced intensity is employed such as in projection exposure techniques where the light is paned through a series of lenses and monochromatic filters. Thus, increased photospeed is particularly important for a resist composition employed in processes where a number of multiple exposures must be made to produce a mask or series of circuit patterns on a substrate. These optimum conditions include a constant development temperature and time in a particular development mode, and a developer system selected to provide complete development of exposed resist areas while minimizing the film thickness loss in the unexposed areas.
Development contrast refers to a comparison between the percentage of film loss in the exposed area of development with the percentage of film loss on the unexposed area. Ordinarily, development of an exposed resist coated substrate is continued until the coating on the exposed area is substantially completely dissolved away and, thus, development contrast can be determined simply by measuring the percentage of the film coating loss in the unexposed areas when the exposed coating areas are removed entirely.
Resist resolution refers to the capability of a resist system to reproduce the smallest equally spaced line pairs and intervening spaces of a mask which is utilized during exposure with a high degree of image edge acuity in the developed exposed spaces. In many industrial applications, particularly in the manufacture of miniaturized electronic components, a photoresist is required to provide a high degree of resolution for very small line and space widths (on the order of one micron or less).
The ability of a resist to reproduce very small dimensions, on the order of one micron or less, is extremely important in the reproduction of large scale integrated circuits on silicon chips and similar components. Circuit density on such a chip can only be increased, assuming photolithographic techniques are used, by increasing the resolution capabilities of the resist.
One approach to increasing the resolution capabilities of a resist is to expose with shorter wavelength light. For example, exposures with mid-UV radiation (about 280-365 nm) typically give on the order of 0.25 .mu.m better resolution than exposures in the near UV region (about 365-459 nm).
One attempt at increasing resolution is provided in U.S. Pat. No. 4,596,763 to DiCarlo et al. (issued June 24, 1986) where a positive photoresist containing a novolak resin and a 1-naphthalene sulfonic acid, 3-diazo-3,4-dihydro-4-oxo-4-benzoyl-1,2,3-benzenetriyl ester photosensitizer is said to provide increased photospeed while retaining or improving the resist contrast in the mid-UV region.
U.S. Pat. No. 4,499,171 to Hosaka et al. (issued Feb. 12, 1985) also discloses positive photoresists which are said to possess excellent sensitivity and yield of residual film thickness. These photoresists contain a novolak resin and a photoactive component which may be a mixture of tri- and diesters of 1,2-naphthoquinonediazide-4-sulfonic acid and 2,3,4-trihydroxybenzophenone. The photoactive component may also contain 2,3,4-trihydroxybenzophenone-1,2-naphthoquinonediazide-4-sulfonic acid monoester.
U.S. Pats. No. 4,377,631 to Toukhy et al. (issued Mar. 22, 1983) and 4,529,682 to Toukhy (issued July 16, 1985) disclose cresol-formaldehyde novolak resins made from mixtures containing meta- and para-cresol, or ortho-, meta-, and para-cresol. These novolak resins may be used with conventional photosensitizers to prepare fast positive photoresist compositions, i.e., those with fast photospeed and good resolution and development contrast. These compositions are particularly sensitive to radiation in the near-ultraviolet to conventional ultraviolet range, i.e., radiation from a wavelength of about 315 to about 450 nanometers. The ratio of ortho-, meta-, and para-cresol in the novolak resins is said to be specific. For instance, when the novolak contains only meta- and para-cresol, a maximum of 53% para-cresol may be used (resins containing 55% para-cresol are specifically stated to be outside the scope of the invention in Table I of U.S. Pat. No. 4,529,682).
Positive photoresists said to have improved photospeed and rate of development are also disclosed in U.S. Pat. No. 4,650,745 to Eilbeck (issued Mar. 17, 1987). The resist contains a novolak resin prepared from, e.g., a 45% meta-cresol, 55% para-cresol mixture, a naphthoquinonediazide sensitizer (e.g., esters of 2,3,4-trihydroxybenzophenone and naphthaquinone-(1,2)-diazide-5-sulfonyl chloride), a dye which absorbs light at a maximum wavelength of from about 330 to about 460 nanometers, and a trihydroxybenzophenone compound.
U.S. Pat. No. 4,731,319 to Kohara et al. (issued Mar. 15, 1988) discloses positive photoresist compositions containing a naphthoquinone diazide sulfonic acid ester photosensitizer and a mixture of cresol novolak resins containing (1) a first cresol novolak resin having a weight-average molecular weight of at least 5000 and produced from an isomeric mixture consisting essentially of 60 to 80% of m-cresol and 40 to 20% of p-cresol, and (2) a second cresol novolak resin having a weight-average molecular weight not exceeding 5000 and produced from an isomeric mixture consisting essentially of 10 to 40% of m-cresol and 90 to 60% of p-cresol. The proportions of the two cresol novolak resins are chosen so that the overall cresol component is comprised of from 30 to 46.5% m-cresol and 70 to 53.5% p-cresol. The only photosensitizers disclosed are 2,3,4-trihydroxybenzophenone naphthaquinone-1,2-diazido-5-sulfonic acid ester; 2,3,4,4'-tetrahydroxybenzophenone naphthoquinone-1,2-diazido-5-sulfonic acid ester; propyl gallate naphthoquinone-1,2-diazido-5 -sulfonic acid and isoamyl gallate naphthoquinone-1,2-diazido-5-sulfonic acid.
European Patent Application Publication No. 0211667 of Hosaka et al. (published Feb. 25, 1987) discloses that monooxymonocarboxylic acid esters, e.g., alkyl 3-alkoxypropionates, are solvents for positive photoresist compositions which impart high storage stability to the resist, i.e., only a small amount of fine particles are formed during storage . The resist may contain novolak resins made from o-, m-, or p-cresol or mixtures thereof (e.g., a mixture of 60% m-cresol and 40% p-cresol) and 1,2-naphthoquinonediazide ester photosensitizers. These sensitizers include 2,3,4-trihydroxybenzophenone-1,2-naphthoquinonediazide-4-sulfonic acid ester, though the 5-sulfonic acid ester isomers are preferred.
European Patent Application Publication No. 0225464 of Collini et al. (published June 16, 1987) discloses composite, two-layer resist structures. The bottom layer is a positive photoresist containing a novolak resin and a diazoquinone compound, e.g., 4',2',3'-dihydroxybenzophenone mixed esters of 1-oxo-2-diazo-naphthalene-5-sulfonic acid [sic]. These photoresists are said to be particularly suitable in the mid ultraviolet range (300-350 nanometers) and the near ultraviolet rane (365 to 450 nanometers).
E. Gipstein et al., "Evaluation of Pure Novolak Cresol-Formaldehyde Resins for Deep U.V. Lithography," J. Eleotrochem. Soc.: Solid-State Science and Technology, Vol. 129, No. 1, pp. 201-205 discusses the effects of novolak resins made from pure o-, m-, or p-cresol on positive photoresists which are exposed to radiation at 254 nm (in the so-called "deep U.V." region). The conventional novolak resins are said to be essentially transparent at 405 nm (i.e., the wavelength of mercury emissions), but have a strong absorption at 254 nm. Thus, the combined absorption of the photoactive component and resin in the resist renders films more than 1 .mu.m thick virtually opaque at 254 nm. Gipstein et al. disclose that novolak resins made from pure o-, m- , or p-cresol are more transparent at 254 nm than the conventional resins, the pure p-cresol novolak being the most transparent.
In a reprint from the Proceedings of SPIE--The International Society for Optical Engineering (Vol. 771, March 1987) entitled "DYNALITH.RTM. Resists for Mid-UV Applications: Formulation Optimization for GaAs Related Processing", R. M. Lazarus and S. S. Dixit discuss the properties of DYNATITH.RTM.X-1605 and X-1608 positive photoresists. DYNATITH.RTM. X-1608 is said to employ a novolak of greater para incorporation than is typical and a sensitizer derived from the following diazo compound: ##STR1## The X-1608 resist has high photospeed and high resolution properties.
Thus, there has long been a need for positive resist compositions which exhibit high photospeeds and good resolution properties when exposed to radiation of relatively short wavelength, e.g., about 248-365 nm. It has now been discovered that positive photoresists comprising novolak resins having a high p-cresol content and a photosensitizer comprising a mixture of mono-, di-, and triesters of 2,3,4-trihydroxybenzophenone and 1,2-naphthoquinone-2-diazo-4-sulfonyl halide provide excellent photospeed and resolution when exposed to radiation in the 248-365 nm range.